This invention relates to a tracking error signal forming circuit and a method for obtaining a tracking error signal, in particular by using a reproduction time difference between signals reproduced by at least two heads having different azimuthal angles.
In a recording/reproduction apparatus, e.g. a magnetic recording/reproduction apparatus (hereinbelow called simply VTR), etc., when information signals recorded on a recording track are reproduced, the tracking control is necessary in order that the play back head reproduces the recorded signals by scanning just on the recording track.
As a method utilized in practice for the tracking control there is known a method, by which a track exclusively used for the tracking control is disposed along the longitudinal direction of the tape, in which control signals are recorded with the frame period on a period, which is an integer times as long as it, at the recording of image signals and the tracking control is effected by using these control signals at the reproduction. However, this method has a drawback that the track exclusively used for the tracking control is necessary and that it is impossible to obtain tracking error signals over the whole region of the recording track.
As another method used in practice there is known a method, by which pilot signals for the tracking control are recorded on the recording track and at the reproduction tracking error signals are obtained by comparing reproduction levels of the pilot signals reproduced from each track adjacent to the main track, which the head is scanning and reproducing. According to this method, since it is possible to obtain tracking error signals over the whole region of the recording track, when the play back head is mounted on an electro-mechanical converting element constituted by a piezo-electric element, etc. and the mechanical position of the head is varied by using the tracking error signals described above, it is possible to construct a control system capable of following bending of the track. However this method has a drawback that, since it is necessary to record the pilot signals, superposing them on the information signal, the bandwidth of the information signal is reduced by the bandwidth corresponding to that of the pilot signals and therefore S/N of the information signal is worsened correspondingly.
In order to remove these drawbacks, a method has been proposed, by which the tracking error signals are obtained over the whole region of the recording track without using pilot signals. The method is disclosed in JP-A-55-150129 and by the method the relationship between the track deviation and the reproduction time difference between reproduced by at least two heads having different azimuth angles is used. Since this invention relates to the method, fundamental conception of the method will be explained below.
FIG. 1 shows magnetization trajectories recorded by two heads having different azimuthal angles. In the figure 101 is a magnetic tape, which is forward in the direction indicated by an arrow 102. 103 and 104 indicate two heads having azimuthal angles, which are different from each other. They scan the tape simultaneously in the direction indicated by an arrow 105. Such heads, i.e. two heads, which scan the magnetic tape simultaneously and have azimuthal angles, which are different from each other, are called below a pair head. 106 and 107 indicate head gaps in the head, respectively. A1, A2, A3, . . . are magnetization trajectories recorded by heads having the same azimuthal angle as the A head indicated by 103 and B1, B2, B3, . . . are magnetization trajectories recorded by heads having the same azimuthal angle as the B head indicated by 104. The pair of tracks A1, B1 and the pair of tracks A2, B2 may be recorded either by a same pair head or by another pair head. This relation is determined by the number of rotations of the rotating cylinder, in which magnetic heads are integrated, and the number of pair heads integrated therein, which can be arbitrarily decided at the design of the device. The signal on each of the magnetization trajectories, e.g. magnetization pattern of the horizontal synchronization signal, etc. is recorded with an angle inclined to the longitudinal direction of the track, as indicated by 108 and 109, i.e. an azimuthal angle.
The method for recording/reproducing information signals by means of a pair head is efficient, when the frequency band of the information signal to be dealt with is great. This is because, since at recording information signals with a constant head scanning speed, the higher the recording frequency of the information signal is, the shorter the wavelength recording it on the magnetic tape is and the shorter it is, the more difficult the practical recording/reproduction is, but it is possible to divide the frequency band of the information signal, the recording wavelength can be set to be substantially long.
FIG. 2 shows the conception, by which the information signal to be recorded is divided into two kinds of signals. FIG. 2-a indicates the original signal to be recorded and FIG. 2-b and FIG. 2-c indicate signals obtained by dividing the original signal, i.e. real recording signals supplied to the pair head. The original signal is divided in time into two portions 201 and 202, e.g. as indicated in the figure, which are elongated in time as indicated by 203 and 204, respectively. That is, it is possible to narrow the frequency band, corresponding to the elongation in time. In FIG. 2, 205, 206, 207, etc. are timing signals, e.g. horizontal synchronization signals. Further the method for dividing the original signal is not restricted to the method, by which the original signal is divided in time, but a method, by which it is divided in frequency, a method, by which it is divided, depending on the kind of signals such as the brightness signal, the color signal, etc., and so forth are conceivable. In any case, in the case where signals having a wide frequency band are to be recorded, the method using a pair head is efficient in practice and inevitable.
FIG. 3 shows 3 positional relations between the recorded magnetization trajectory and the play back scanning head. In this figure 301, 302 and 303 indicated by broken lines indicate pair heads, each of which consists of an A head and a B head. Each of the pair heads scans a trajectory in the direction indicated by an arrow 304. A1 and B1 are magnetization trajectories recorded by a pair head and signals indicated by 305 to 310 represent recording positions of horizontal synchronization signals. The position of the pair head with respect to the recorded magnetization trajectories is deviated towards left on the sheet of the FIG. 3-a, on-track in FIG. 3-b and deviated towards right in FIG. 3-c. When a recording track is play-back-scanned with heads having such relative positional relations, even if signals are recorded at a same timing, they are reproduced with different timings. For example, in FIG. 3-a, the timing, where the horizontal synchronization signal 305 is reproduced by the head A, is retarded with respect to the timing, where the horizontal synchronization signal 306 is reproduced by the head B; in FIG. 3- b the reproduction timings of the two horizontal synchronization signals are equal; and finally in FIG. 3-c the timing, where the horizontal synchronization signal 309 is reproduced by the head A, is advanced with respect to the timing, where the horizontal synchronization signal 310 is reproduced by the head B. Consequently track deviations can be known by examining the time difference of the horizontal synchronizaton signals reproduced by the heads A and B. Further the signal, for which the time difference is examined, is not restricted to the horizontal synchronization signal, but it may be another specified signal. However this invention will be explained below, supposing that the specified signal is the horizontal synchronization signal.
FIG. 4 is a graph showing the relation between the track deviation and the reproduction time difference between the horizontal synchronization signals reproduced by the heads A and B, in which the abscissa indicates the track deviation, the position indicated by 0 being the on-track position. The direction of the deviation towards right and left corresponds to the direction of the deviation of the head with respect to the recording track on the sheet indicated in FIG. 3. The ordinate indicates the reproduction time difference between the horizontal synchronization signals reproduced by the heads A and B and the timing, where the time difference is 0, corresponds to the on-track position. Further it is supposed that the direction of the deviation is positive, when the timing of the horizontal synchronization signal reproduced by the head A is retarded with respect to that reproduced by the head B. At this time the relation between the track deviation and the time difference between the reproduced signals is represented by a line 401. As it is clear from FIG. 4, if the tracking control circuit is so constructed that the time difference indicated by the ordinate tends to be zero, the play back head play-back-scans the recorded track always on-track.
However the method, by which the tracking control is effected by detecting the time difference between the signals reproduced by the pair head, is difficult, only when the center positions of the head gaps of the heads A and B are strictly in accordance with each other, and otherwise it gives rise to track deviations. This will be explained below.
FIG. 5 illustrates 3 mounting states of the heads A and B. In the figure 501, 503 and 505 indicate the head A and 502, 504 and 506 show the head B. The scanning direction of the heads is indicated by arrows 507, 508 and 509. 512, 513 and 514 represent members for mounting the heads, which are movable members constituted by piezo-electric elements, etc. The heads A and B are mounted so as to be deviated from each other in the direction perpendicular to the scanning direction. The gap in each of the head is indicated by a full line drawn aslant in each of the heads, as indicated by 510 and 511.
FIG. 5-a shows the correct mounting state, in which the center points 515 and 516 of the gaps of the heads are on a line 517, which is perpendicular to the scanning direction of the heads. That is, there are no deviations in the scanning direction. In a magnetization trajectory recorded by using such heads signals havng a same timing are recorded side by side in the direction perpendicular to the longitudinal direction of the recorded track.
FIG. 5-b shows an incorrect mounting state, in which the heads are mounted so that the center points of the gaps thereof are deviated by an amount indicated by 518 from each other in the scanning direction. In a magnetization trajectory recorded by using such a head signals having a same timing are recorded, deviated by the amount indicated by 518 from each other in the longitudinal direction of the recorded track.
FIG. 5-c illustrates another mounting method of the heads, in which the heads are mounted so that the center points of the gaps thereof are shifted by an interval 518 corresponding to the recording wavelength of the horizontal synchronization signal and in addition the amount of the deviation 519 in the height of the heads produced by the shift of the head is corrected by a height correcting member 520. It is well known that it is possible to locate the recording positions of the horizontal synchronization signals side by side in the longitudinal direction of the recorded track by means of the heads thus constructed. However, also in this case, unless the interval 518 is set so as to be precisely equal to the wavelength of the horizontal synchronization signal or a length, which is an integer times as long as the wavelength, the condition is identical to that explained, referring to FIG. 5-b.
FIG. 6 shows the relation between the recorded magnetization trajectory and the play back head, in the case where a magnetization trajectory recorded by a pair head, in which the mounting positions of the heads are deviated from each other, is reproduced by another pair head having also deviated heads. 601 and 602 represent heads A and B, respectively, and 603 and 604 indicate recording positions of the horizontal synchronization signals. The figure shows a state, in which the pair head scans the recorded track, when it is on-track. At this time the time difference between reproduction timings of the two horizontal synchronization signals reproduced by the heads A and B corresponds to a difference between distances indicated by 605 and 606. That is, even if the pair head is on-track, a time difference is produced between the reproduced signals.
FIG. 7 shows the relation between the track deviation and the time difference between the reproduced signals, in which the abscissa and the ordinate have the same meanings as those explained for FIG. 4 and 401 is the characteristic line indicated in FIG. 4. That is, for this characteristic line the time difference zero corresponds to the on-track position. On the other hand the characteristic line indicated by 701 is that obtained, in the case where the relation between the heads and the recorded magnetization track is that indicated in FIG. 6. That is, in this case, the time difference is not zero at the on-track position. If the control were effected so that the time difference is zero at this time, the control system would be stabilized at the track position indicated by 702.
The mounting precision of the heads A and B is determined by the mechanical precision at the head mounting and fluctuations are always produced. Consequently it can be said that the relation between the recording position of the signal and the play back pair head is generally that indicated in FIG. 6. Furthermore, since the relation between the distances indicated by 605 and 606 in FIG. 6 is different from deck to deck, unless this problem is resolved, the method disclosed in JP-A-54-57287 cannot be utilized in practice.